(1) Field of the invention
The present invention relates to a system and method for controlling ignition timing applicable to an internal combustion engine having cylinder banks such as a V-type or flat-type engine.
(2) Background of the art
Various ignition timing control systems and methods have been proposed using microcomputers.
One of the previously proposed ignition timing control systems is exemplified by Japanese Patent Application First Publication No. Showa 55-112861 published on Sept. 1, 1980.
The previously proposed ignition timing control system disclosed in the above-identified Japanese Patent Application First Publication includes a control unit having a microcomputer. The control unit includes a memory having an ignition timing map allocated according to an engine revolution speed N and a basic fuel injection quantity T.sub.p. The engine revolution speed N is derived from an output signal of a crank angle sensor. The basic fuel injection quantity T.sub.p is derived on the basis of the engine revolution speed N and intake air quantity Q determined from the output signal of an airflow meter. Then, an optimum ignition timing is generally derived by comparing the ignition timing map with the signals indicating the present engine revolution speed N and basic fuel injection quantity T.sub.p . An ignition device receives an ignition signal derived at a time corresponding to the optimum ignition timing and ignites an air/fuel mixture supplied to a corresponding cylinder at the time described above.
In general, in a V-type internal combustion engine having double overhead camshafts (DOHC, also called; twin cam), a pair of cam shafts are arranged for suction and exhaust valves on each cylinder head of left and right cylinder banks, respectively. Each cam shaft receives rotational force from a crankshaft via a timing chain and cam chain. A crank angle sensor is installed on one of the cam shafts for the exhaust valves on one side (for example, right side) of the respective banks so as to detect the cam shaft revolution of the right side exhaust valve bank as the engine revolution speed and also to detect a crank angular position within 1.degree.. The control unit determines the ignition timing on the basis of the respective signals derived from the crank angle sensor.
However, since, in the previously proposed ignition timing control system disclosed in the above-identified Japanese Patent Application First Publication, the engine revolution speed and so on is derived from the crank angle sensor by the number of cam revolutions per unit of time (unit angle). That is to say, although the crankshaft is always rotated with uniform motion, each cam shaft generates a non-uniform rotation (i.e., variation in the rotational motion occurs) due to variations in a tension of the timing chain and valve spring reactions of each suction and exhaust valve and due to deviations in cam profiles inherent to the cam shafts.
Especially, in V-type six cylinder engines, a timing at which the exhaust valves are lifted does not overlap over a crank angular range in the vicinity to the crank angle of 240.degree., as shown in FIG. 5, but is continued over the crank angular range.
For example, for the exhaust strokes of second, fourth, and sixth cylinders in the right side bank, the exhaust stroke of the second cylinder is started at the same time when the exhaust stroke of the fourth cylinder is ended. Furthermore, the exhaust stroke of the sixth cylinder is started at the same time when the exhaust stroke of the fourth cylinder is ended. Hence, a characteristic graph of valve lift timings indicates a continous waveform. On the other hand, for first, third, and fifth cylinders on the right bank, a similar continous waveform charactersitic is exhibited.
For suction valves, the same result is obtained.
In this way, the cam shafts generate such variations of rotational motion and such variations are detected by means of the crank angle sensor and input to the control unit.
Consequently, variation in the ignition timing occurs between left and right banks. In general, more retardation of the ignition timing occurs in the cylinders of the one bank in which the crank angle sensor is installed than that in the other bank in which no crank angle sensor is installed.
Therefore, the accuracy of the ignition timing is reduced, an engine output torque is accordingly reduced, and a large difference occurs in exhaust gas temperature between the left and right banks. Furthermore, a controllability of an engine knock avoidance is worsened. It is noted that a normal ignition timing is set in a vicinity to 25.degree. before piston top dead center (BTDC 25.degree.) in an explosion stroke (refer to FIG. 5).